A turbofan gas turbine engine comprises a fan carrying a plurality of circumferentially spaced radially extending fan blades arranged to rotate within a fan duct defined by a fan casing. The fan casing is supported from a core engine casing by struts extending radially across the fan duct from the fan casing to the core engine casing and the engine is carried by a pylon which is secured to the core engine casing. The pressure non-uniformity is particularly strong in the fan duct due to the pylon and struts which extend radially across the fan duct and also due to a fairing for a radial drive shaft which extends radially across the fan duct and which may be located at the bottom of the gas turbine engine. These obstacles, the pylon, the struts and the fairing, generate circumferentially varying pressure levels, which may result in fan blade forced response excitation, noise generation and an increase in aerodynamic losses.
Conventionally fan outlet stator vanes are arranged axially between the pylon and the fan blades and the fan outlet stator vanes have been arranged to minimise the forcing on the fan blades.
It is known to arrange the fan outlet stator vanes such that some of them are over cambered and some of them are under cambered.
It is known from our UK patent GB1291235 to arrange the leading edges of the fan outlet stator vanes in a helical arrangement between struts.
It is known from our published UK patent application GB2046849A to arrange the fan outlet stator vanes axially upstream of the struts and to provide an asymmetric shape on the leading edge of the strut.
It is known from our published European patent application EP0942150A2 to arrange the fan outlet stator vanes between the struts, to arrange all the leading edges in the same plane and to vary the circumferential position of the fan outlet stator vanes between the struts.
It is also known from published International patent application WO9301415A to arrange alternate vanes at a first axial position and the remainder of the vanes at a second axial position.